Electrophoretic display

ABSTRACT

The invention relates to an ambient light electrophoretic display comprising a plurality of cells containing a suspension of charged pigment particles in a light-transmissive fluid. Each cell comprises a light-transmissive front window, a nonobstructing collecting electrode and counter electrode disposed in the cell, and a panel having a reflective or absorbing surface.

TECHNICAL FIELD

The present invention relates to an electrophoretic display, and inparticular relates to a reflective electrophoretic display.

BACKGROUND OF THE INVENTION

An electrophoretic display generally comprises a suspension of chargedpigment particles colloidally dispersed in a liquid of matching specificgravity contained in a cell comprising two parallel and transparentconducting electrode panels. The charged particles are transportedbetween the electrode panels under the influence of an electric field.In one embodiment of an electrophoretic display disclosed in Hou, U.S.Pat. No. 5,298,833, black particles are suspended in a back-lightedclear medium. When the black particles are moved against the fronttransparent electrode, there is displayed a black image contrastedagainst a light background. Unfortunately, such a display requires aback light, and therefore it is not desirable for notebook computers dueto power and weight requirements of the backlight.

Dalisa, "Electrophoretic Display Technology", Proceedings of S.I.D.,Vol. 18, No. 1, 1977, pp. 43-50; and "Electrophoretic Displays", Topicsof Applied Physics, Vol. 40, 1980, pp. 213-232, discloses a reflectiveelectrophoretic display (e.g., no back light) comprising charged pigmentparticles dispersed in a dyed liquid of contrasting color enclosed in acell comprising parallel and transparent conducting front and rearelectrode panels. When the pigment is packed on the front electrodepanel, the color of the pigment will be seen by the observer withambient light. When the pigment is on the rear electrode panel, theambient room light is absorbed and scattered by the dyed liquid and thecolor of the dye is observed. Because the display is passive andrequires no back light, the display consumes very little power. Toachieve the greatest contrast, light particles with large scatteringcoefficients such as titanium dioxide are suspended in a dark dyedmedium. Displays utilizing this approach, however, do not produce highlycontrasted images and generally have low reflectivity. When their dyeconcentration is low, the suspension, and hence the background, appearsgrayish. When their dye concentration is high, the light state is notbright because the dark dye is easily noticed in the spaces between theparticles and in the spaces between the particles and the viewing plate.

Therefore, there is a continuing need in the art for an electrophoreticdisplay having low power consumption and good contrast and brightness.

It is therefore an object of the present invention to provide animproved electrophoretic display having low power consumption and goodimage contrast. Other objects and advantages will become apparent fromthe following disclosure.

SUMMARY OF THE INVENTION

The present invention relates to an ambient light electrophoreticdisplay comprising a plurality of cells containing a suspension ofcharged pigment particles in a light-transmissive fluid. Each cellcomprises a light-transmissive front window, a nonobstructing collectingelectrode disposed in the cell, a counter electrode disposed in thecell, and a panel which has a light-reflective or absorbing surface. Thenonobstructing collecting electrode is preferably disposed on the panelwhich has a light-reflective or absorbing surface and is preferablylocated at the rear of the cell. In the collected state with thepigments collected by the collecting electrode, incident ambient lightis reflected off or absorbed by the rear surface of the cell.Preferably, the light is reflected off the rear panel and back to theviewer.

A more thorough disclosure of the present invention is presented in thedetailed description which follows and from the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of an electrophoretic display cell of thepresent invention in the dark-image state.

FIG. 2 is a sectional view of an electrophoretic display cell of thepresent invention in the light-image state.

FIG. 3 is an alternative embodiment of an electrophoretic display cellof the present invention.

FIG. 4 is another alternative embodiment of an electrophoretic displaycell of the present invention.

FIG. 5 is another alternative embodiment of an electrophoretic displaycell of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to an ambient light electrophoreticdisplay comprising a plurality of cells, each cell containing asuspension of charged pigment particles in a light-transmissive fluid.Each cell comprises a light-transmissive front window, a counterelectrode, a nonobstructing collecting electrode, and a panel which hasa light-reflective or absorbing surface. The panel covers substantiallyall of the horizontal (lateral) area of the cell. In a preferredcollected state, the pigment particles are collected on thenonobstructing collecting electrode which enables ambient light to bereflected off the panel and back to the viewer. The cell operateswithout need of any contiguous artificial electric light source (e.g.,backlight).

The pigment particles preferably have stable properties and singlepolarityfor better contrast, resolution, and dispersibility. Preferably,the pigment particles are opaque. The pigment particle can be black orcoloredsuch as white, red, green, or the like. Suitable pigmentparticles include:

Inorganic: Cadmium Red

Cadium sulfo-selenide (black)

Chromium oxide (green)

Iron oxides (black)

Iron oxides (red)

Lead chromate (yellow)

Manganese dioxide (brown)

Silicon monoxide (reddish brown)

Sulfur (yellow)

Vermilion Red

Carbon black

Titanium oxide

Organic: Anthracene (fluorescent blue)

Anthracene (fluorescent yellow)

Phthalocyanine Blues

Phthalocyanine Greens

Suitable opaque pigment particles include:

Stirling NS N 77Y (Pigment Black 7)

Cabot Mogul L (black)

Carbon Black

Monastral® Green G (C.I. Pigment Green 7)

Monastral® Blue G (C.I. Pigment Blue 15)

Toluidine Red Y (C.I. Pigment Red 3)

Quindo® Magenta (Pigment Red 122)

Indo® Brilliant Scarlet (Pigment Red 123)

Toluidine Red B (C.I. Pigment Red 3)

Watchung® Red B (C.I. Pigment Red 48)

Permanent Rubine F6B13-1731 (Pigment Red 184)

Hansa® Yellow (Pigment Yellow 98)

Dalamar® Yellow (Pigment Yellow 74)

Toluidine Yellow G (C.I. Pigment Yellow)

Monastral® Blue B (C.I. Pigment Blue 15)

Monastral® Green B (C.I. Pigment Green 7)

Pigment Scarlett (C.I. Pigment Red 60)

Anric Brown (C.I. Pigment Brown 6)

Other suitable pigment particles will be known to those skilled in theart such as those disclosed in U.S. Pat. Nos. 5,200,289 and 4,631,244.

The pigment particles are colloidally suspended in a light-transmissivefluid, preferably clear fluid. The fluid is preferably dielectric andsubstantially free of ions. The fluid preferably has minimum solventaction on the pigments and a specific gravity about equal to the pigmentparticles. Suitable solvents include silicone fluid such as (dimethylpolysiloxane), and hydrocarbons such as decane, dodecane, N-tetradecane,xylene, Sohio odorless solvent (a kerosene fraction available from ExxonCompany), toluene, hexane and Isopar® G, H, K, L, M, and V and Norpar®12, 13, and 15 (branched and linear saturated aliphatic hydrocarbonsavailable from Exxon Company).

The pigments are dispersed in the fluid to form a stable colloidalsuspension. Optionally, other components can be added to the suspensionsuch as charge control additives, dispersants, and surfactants toimprove the performance of the suspension. Suitable additives includesodium dioctylsulfonsuccinate, zirconium octoate, and metal soaps suchas lecithan, barium petronate, calcium petronate, alkyl succinimide,iron naphthenate, and polyethylene glycol sorbitan stearate.

Referring to FIGS. 1 and 2, there is shown a preferred embodimentelectrophoretic display cell in accordance with the present invention.Thecell generally comprises a front light-transmissive counter electrode2 (functioning as the front window), a horizontal rear panel 4 having alight-reflective or absorbing surface 6 facing the electrode 2, and anonobstructing collecting electrode 8 disposed in the cell and spacedfromthe front electrode. The pigment suspension 10 is disposed in thespace between the front electrode 2 and the rear panel 4.

The light-transmissive counter electrode of the embodiment in FIG. 1 canbesuitably formed by disposing onto a glass plate a layer of conductive,light-transmissive material such as indium tin oxide.

The surface 6 of rear panel 4 preferably reflects incident ambient lightback through the cell and the light-transmissive front panel. Thereflective surface is preferably silvered to enhance the degree ofreflectivity. Alternatively, the panel can have a reflective coloredsurface such as a white-reflective surface. Suitably, the rear panel 4canbe formed from aluminum, chromium, or nickel. The reflective surfacepreferably has a reflectivity of greater than about 60%, preferablygreater than about 85%, and more preferably greater than 90%. If thepanelis light absorbing, it can have a black-absorbing surface such as ablack-painted surface.

The nonobstructing collecting electrode 8 is sized and/or positioned sothat when the electrode has collected and is coated with the pigmentparticles, the electrode and pigment particles do not unacceptablyinterfere with the transmission of incident light through thelight-transmissive fluid. (Substantially all of the incident light istransmitted through the cell.) For example, the electrode can be made asmall size (e.g., square or pedestal shaped or narrow line) andpositionedalong the side of the rear panel (line electrode) or at thecorner (square)such as shown in FIG. 1. Alternatively, the electrode canbe disposed vertically along the side of the cell out of the line ofsight of the viewer.

In FIG. 1, in the dark or opaque state (noncollected state), the pigmentparticles are generally uniformly dispersed in the suspension betweenthe front counter electrode 2 and rear reflective panel 4. The incidentambient light is either absorbed by the pigment particles to create ablack image or alternatively reflected to create a colored image. InFIG. 2, the cell is in the collected state. The pigments 12 in thesuspension are attracted to and collected on the nonobstructingcollecting electrode 8. This permits the transmission of incidentambient light through the cell and reflects off the reflective surface 6and back through the cell to create a light image. The light image hasgood contrast against the black or pigment-colored image of FIG. 1. Ifthe cell in FIG. 2 had a light-absorbing surface 6 on panel 4, the stateshown in FIG. 2 would be adark image. The process for makingelectrophoretic display cells is known in the art such as disclosed inU.S. Pat. Nos. 4,218,302 and 4,650,288; and Dalisa, "ElectrophoreticDisplays", Top. Appl. Phys., Vol. 40,pp.213-232, Display Devices (1980),the disclosures of which are incorporated herein by reference.

Referring to FIG. 3, there is shown an alternative embodiment of thedisplay cell of the present invention comprising a light-transmissivefront window 20, vertical collecting electrodes 22 disposed on the sideofthe cell between the front and rear of the cell, rear panel 24 havinga reflective surface, and pedestal-shaped counter electrode 26. In thedark state, the pigment particles are generally uniformly dispersed inthe suspension.

Referring to FIG. 4, there is shown another alternative embodiment ofthe display cell of the present invention comprising light-transmissivefront counter electrode panel 30, reflecting horizontal panel segments32, and collecting electrode 34. In the dark state, the pigments aredisposed on the front panel electrode and, in the light state, thepigments are disposed on the collecting electrode behind the reflectivepanel segments 32.

Referring to FIG. 5, there is shown another alternative embodiment ofthe display cell of the present invention. The cell comprisespedestal-shaped collecting electrode 40, a light-transmissive counterelectrode 42 disposed over a rear panel having a reflecting surface, anda third light-transmissive front electrode 44. The dark-image state canbe achieved by disposing the particles on the front electrode or counterelectrode, or dispersing the particles in the suspension. Alternatively,the counter electrode can have a light-reflecting surface and functionas the counter electrode and rear panel.

The following example is a detailed description of a display of thepresentinvention. The details fall within the scope of, and serve toexemplify, the more general description set forth above. The example ispresented forillustrative purposes only, and are not intended as arestriction on the scope of the invention.

EXAMPLE

ELECTROPHORETIC DISPLAY CELL

A 10 mm square cell was constructed comprising (i) a frontlight-transmissive electrode made of indium tin oxide (ITO), (ii) a rearglass panel having a 3 mm ITO strip along one side, and (iii) 1.1 mmteflon spacers between the front electrode and rear glass panel. Alight-reflecting white paper was placed behind the rear glass panel. Thecell was filled with Versatec Black liquid toner diluted 1:11 withVersatec solvent. The dispersed pigment suspension gave a dark image tothe cell. The electrodes were then charged and pigment was collected onthe rear strip electrode to provide a white-reflective image.

Although this invention has been described with respect to specificembodiments, the details thereof are not to be construed as limitationsfor it will be apparent that various embodiments, changes, andmodifications may be resorted to without departing from the spirit andscope thereof, and it is understood that such equivalent embodiments areintended to be included within the scope of this invention.

What is claimed is:
 1. An ambient light electrophoretic displaycomprising a plurality of cells containing a suspension of chargedpigment particles in a light-transmissive fluid, each cellcomprising:(a) a light-transmissive front window; (b) a nonobstructingcollecting electrode; (c) a counter electrode; and (d) a panel having asurface which is light-reflective to reflect incident light back throughthe cell and the front window.
 2. The display of claim 1 wherein thepanel is disposed at the rear of the cell.
 3. The display of claim 1wherein the counter electrode is light transmissive.
 4. The display ofclaim 2 wherein the counter electrode is the panel.
 5. The display ofclaim 3 wherein the reflective surface is silvered.
 6. The display ofclaim 5 wherein the pigment particles are opaque.
 7. An electrophoreticdisplay comprising a plurality of cells containing a suspension ofcharged pigment particles in a light-transmissive fluid, each cellcomprising:(a) a light-transmissive front electrode; (b) anonobstructing collecting electrode; and (c) a rear panel having areflective surface to reflect the incident light back through cell andthe front electrode.
 8. The display of claim 7 wherein the collectingelectrode is disposed on the rear panel.
 9. The display of claim 8wherein the reflective surface is silvered.
 10. The display of claim 9wherein the pigment particles are opaque.
 11. An ambientlight,reflective, electrophoretic display without backlight comprising aplurality of cells containing a suspension of charged pigment particlesin a light-transmissive fluid, each cell comprising:(a) alight-transmissive front window; (b) a nonobstructing collectingelectrode disposed at the side of the cell; (c) a counter electrode; and(d) a panel having a surface which is light-reflective or absorbing.